A series of ternary organic photovoltaics (OPVs) are fabricated with one wide bandgap polymer D18-Cl as donor, and well compatible Y6 and Y6-1O as acceptor. The open-circuit-voltage (VOC ) of ternary OPVs is monotonously increased along with the incorporation of Y6-1O, indicating that the alloy state should be formed between Y6 and Y6-1O due to their excellent compatibility. The energy loss can be minimized by incorporating Y6-1O, leading to the VOC improvement of ternary OPVs. By finely adjusting the Y6-1O content, a power conversion efficiency of 17.91% is achieved in the optimal ternary OPVs with 30 wt% Y6-1O in acceptors, resulting from synchronously improved short-circuit-current density (JSC ) of 25.87 mA cm-2, fill factor (FF) of 76.92% and VOC of 0.900 V in comparison with those of D18-Cl : Y6 binary OPVs. The JSC and FF improvement of ternary OPVs should be ascribed to comprehensively optimal photon harvesting, exciton dissociation and charge transport in ternary active layers. The more efficient charge separation and transport process in ternary active layers can be confirmed by the magneto-photocurrent and impedance spectroscopy experimental results, respectively. This work provides new insight into constructing highly efficient ternary OPVs with well compatible Y6 and its derivative as acceptor.

Approaching 18% efficiency of ternary organic photovoltaics with wide bandgap polymer donor and well compatible Y6 : Y6-1O as acceptor.

Perovskite quantum-dot-based light-emitting diodes (QLEDs) possess the features of wide gamut and real color expression, which have been considered as candidates for high-quality lightings and displays. However, massive defects are prone to be reproduced during the quantum dot (QD) film assembly, which would sorely affect carrier injection, transportation and recombination, and finally degrade QLED performances. Here, we propose a bilateral passivation strategy through passivating both top and bottom interfaces of QD film with organic molecules, which has drastically enhanced the efficiency and stability of perovskite QLEDs. Various molecules were applied, and comparison experiments were conducted to verify the necessity of passivation on both interfaces. Eventually, the passivated device achieves a maximum external quantum efficiency (EQE) of 18.7% and current efficiency of 75 cd A−1. Moreover, the operational lifetime of QLEDs is enhanced by 20-fold, reaching 15.8 h. These findings highlight the importance of interface passivation for efficient and stable QD-based optoelectronic devices. Perovskite quantum-dots are promising candidates for light-emitting diodes but the defects limit the device performance. Here Xu et al. show a passivation strategy to reduce the defect density at both interfaces, which increases the external quantum efficiency (EQE) and lifetime by more than 2-fold and 20-fold, respectively.

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A bilateral interfacial passivation strategy promoting efficiency and stability of perovskite quantum dot light-emitting diodes

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon related to the hysteresis in the current–voltage curve. The apparent negative capacitance remains elusive in the impedance analysis of metal halide perovskite solar cells. Here Ebadi et al. show that it can be attributed to slow transients in the injection current instead of classical capacitive effect, i.e. charge accumulation.

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Origin of apparent light-enhanced and negative capacitance in perovskite solar cells

Side-chain engineering has been shown to be an important strategy to optimize Y-series nonfullerene acceptors (NFAs). Most previous reports were focusing on changing the branching positions and size of the alkyl side chains on Y6. In this paper, we investigate the influence of the orientation of side chains on the properties of NFAs and the performance of the organic solar cells (OSCs). Three isomeric NFAs named o-BTP-PhC6, m-BTP-PhC6, and p-BTP-PhC6 are designed by changing the substitution positions and thus orientations of the side chains attached to the central core. Our studies show that the optimal side-chain orientation can be achieved by the meta-positioned hexylphenyl group (of the m-BTP-PhC6 molecule), which introduces significant beneficial effects on optical absorption, intermolecular packing and phase separation of the NFAs. By pairing a donor polymer PTQ10 with m-BTP-PhC6, device efficiencies of 17.7% can be achieved, which is among the best values for PTQ10-based nonfullerene OSC devices so far. These results reveal that regulating side-chain orientations of Y-series NFAs is a promising strategy to achieve favorable morphology, and high charge mobility and solar cell performances.

Fine-tuning of side-chain orientations on nonfullerene acceptors enables organic solar cells with 17.7% efficiency

Light-emitting diodes based on halide perovskites have undergone rapid development in recent years and can now offer external quantum efficiencies of over 23%. However, the practical application of such devices is still limited by a number of factors, including the poor efficiency of blue-emitting devices, difficulty in accessing emission wavelengths above 800 nm, a decrease in external quantum efficiency at high current density, a lack of understanding of the effect of the electric field on mobile ions present in the perovskite materials, and short device lifetimes. Here we review the development of perovskite light-emitting diodes. We examine the key challenges involved in creating efficient and stable devices, and consider methods to alleviate the poor efficiency of blue-emitting devices, leverage emission in the long infrared region and create spin-polarized light-emitting diodes. This Review examines the development of perovskite light-emitting diodes, exploring the key challenges involved in creating efficient and stable devices. 

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Perovskite light-emitting diodes

Wide bandgap polymer donor PM6 and narrow bandgap polymer acceptor PY-IT were selected to construct all-polymer solar cells (all-PSCs) with layer-by-layer (LbL) or bulk heterojunction (BHJ) structure. The additive 1-chloronaphthalene...

Achieving 15.81% and 15.29% efficiency of all-polymer solar cells based on layer-by-layer or bulk heterojunction structure

Ultraviolet (UV) narrowband photodetectors play a critical role in missile detection, flame monitoring, optical communication, etc. It is a great challenge to realize UV narrowband organic photodetectors due to wide photo‐harvesting property of organic materials, especially for photomultiplication type organic photodetectors (PM‐OPDs). In this work, a smart strategy is proposed to achieve UV narrowband response by coupling Fabry−Pérot microcavity with PM‐OPDs. PM‐OPDs are realized by using poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C71‐butyric acid methyl ester (100:1, w/w) as active layers, exhibiting broadband response range covering from UV–vis region. Series of optical microcavities consisting of Ag/LiF/Ag with UV spectral selectivity are prepared, which are employed to couple with the PM‐OPDs for achieving UV narrowband response. The UV spectral selectivity of optical microcavity can be optimized by tuning the thickness of spacer layer and mirror layers, which can further regulate the photogenerated electron distribution near Al electrode to optimize the external quantum efficiency (EQE) spectra of the PM‐OPDs coupled with optical microcavity. The optimized PM‐OPDs coupled with optical microcavity exhibit EQE of 9300% at 350 nm and narrowband response with 33 nm full‐width at half‐maximum under −15 V bias. This work indicates that PM‐OPDs coupled with optical microcavity should be an efficient strategy for achieving UV narrowband response.

Ultraviolet Narrowband Photomultiplication Type Organic Photodetectors with Fabry−Pérot Resonator Architecture

Layer‐by‐layer organic photovoltaics (LbL‐OPVs) are fabricated using wide bandgap polymer D18 with narrow photon harvesting range in visible light region and narrow bandgap small molecular N3 with strong near‐infrared photon harvesting; the only difference is D18 layer thickness adjusted by spin coating speed. A 15.75% power conversion efficiency (PCE) is obtained from the LbL‐OPVs with D18 layer prepared under 7000 round per minute of spin coating condition; the corresponding D18/N3 layers have a 52.06% of average visible transmittance (AVT) in the spectral range from 370 to 740 nm. Based on the optimized D18/N3 layers, semitransparent LbL‐OPVs are built with 1 nm Au/(10, 15, 20 nm) Ag as the top electrode. The PCE and AVT of semitransparent LbL‐OPVs can be simultaneously adjusted by altering Ag layer thickness due to its variable reflectance and conductivity of top electrode dependence on Ag layer thickness. The PCE/AVT of 12.58%/22.81%, 13.80%/15.09%, and 14.85%/9.48% can be individually achieved from the semitransparent LbL‐OPVs with 10, 15, or 20 nm‐thick Ag layer, which should be among the highest values of semitransparent OPVs based on bulk heterojunction or LbL structures. Adjusting donor layer thickness may be an effective method to construct efficient semitransparent LbL‐OPVs.

Efficient Semitransparent Layer‐by‐Layer Organic Photovoltaics via Optimizing Wide Bandgap and Narrow Absorption Polymer Layer Thickness

Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA2PbI4:PEA2SnI4, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA2PbI4/PEA2SnI4 heterostructure when shearing-away PEA2SnI4 film onto the surface of PEA2PbI4 film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA2PbI4:PEA2SnI4 interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites. Forming charge transfer excitons (CTEs) exclusively within perovskite structures remains as an unexplored issue. Here, the authors report the establishment of CTEs for demonstrating broad light emission within quasi-2D perovskite heterostructures, presenting “intermolecular-type” excited states.

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Establishing charge-transfer excitons in 2D perovskite heterostructures.

Layer-by-layer polymer solar cells (LbL-PSCs) were prepared with PNTB6-Cl as donor and Y6 as acceptor by sequential spin-coating method. Two solvent additives DPE and DFB were individually incorporated into PNTB6-Cl...

Xixiang Zhu

Papers

Achieving 15.81% and 15.29% efficiency of all-polymer solar cells based on layer-by-layer or bulk heterojunction structure

Ultraviolet Narrowband Photomultiplication Type Organic Photodetectors with Fabry−Pérot Resonator Architecture

Efficient Semitransparent Layer‐by‐Layer Organic Photovoltaics via Optimizing Wide Bandgap and Narrow Absorption Polymer Layer Thickness

Establishing charge-transfer excitons in 2D perovskite heterostructures.

Achieving 17.5% efficiency of polymer solar cells via donor and acceptor layered optimization strategy